Video game consoles went from the Atari to the PlayStation 5, and the way scientists picture enzymes has had an upgrade too! The newer model fits the evidence better, and it’s the one the MCAT expects you to use.
On this page, we’ll walk you through enzyme models and classification, from how an enzyme grabs its substrate to the seven types of enzymes and their functions. We’ll finish with the cofactors and coenzymes many enzymes need. Let’s get started!
Enzyme Models & Classification on the MCAT: What to Know
Enzymes show up in both science sections! The AAMC content outline lists enzyme classification by reaction type, the active site and induced fit models, and cofactors, coenzymes and water-soluble vitamins in Bio/Biochem, and again in Chem/Phys. Our AAMC outline checklist lays out every topic.
Questions rarely ask you to recite a definition. More often, a passage shows a reaction and expects you to name the enzyme class or explain how binding works, so that’s what we’ll practice! These are the three things to learn.
- The two enzyme models, lock and key and induced fit
- The seven classes of enzymes and the reaction each one catalyzes
- Cofactors and coenzymes, the helpers many enzymes can’t work without
Free cheat sheet
The Enzymes Cheat Sheet
Lock and key against induced fit, the inhibition types, optimal pH and temperature, and Michaelis-Menten. Four pages.
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Lock and Key vs Induced Fit: The Two Enzyme Models
An enzyme binds its substrate in the active site, a pocket shaped and charged to hold that molecule (or a small set of similar ones). The two models describe how that fit comes about.
The lock and key model
Emil Fischer proposed the lock and key model in 1894. The active site (the lock) already has the shape of the substrate (the key), so the substrate slots in and the enzyme’s shape doesn’t change. Neat and tidy!
The model’s weak point is that it treats binding as rigid. Research shows enzymes change their shape (conformation) as they bind, and this model doesn’t have room for that.
The induced fit model
Daniel Koshland proposed the induced fit model in 1958. The active site starts out only partly complementary to the substrate. As the two interact, binding induces a change in the enzyme’s conformation, until the active site wraps closely around the substrate.
Hexokinase is the classic example you’ll see. When glucose binds, the enzyme’s two lobes close around it, which keeps water out of the active site so the phosphate from ATP goes to glucose.
Picture a memory foam mattress. It’s flat when you lie down, and over the night it takes on the shape of your body. The active site molds itself around its substrate in a similar way!
Induced fit still keeps enzymes specific. Your memory foam bed molds to you or a family member, and you probably wouldn’t let a random stranger sleep in it. The wrong substrate doesn’t trigger the right shape change, so no reaction happens.
Induced fit. Research supports it, and it explains the conformational changes that happen during binding. If a question asks which model accounts for those changes, induced fit is your answer!
Our enzyme structure and function notes cover the active site and how enzymes lower activation energy, if you’d like a refresher first.
The 7 Types of Enzymes and Their Functions
Enzyme classification is based on the type of reaction an enzyme catalyzes.
Six classes were recognized starting in 1961, and the IUBMB enzyme naming committee added a seventh, translocases, in 2018. Each class has an EC (Enzyme Commission) number from 1 to 7, so there’s a neat order to them!
This chart gives each class with a few examples you’ll meet in biochemistry passages.
Ligase, Isomerase, Lyase, Hydrolase, Oxidoreductase and Transferase are the six classic classes. Add translocases as the seventh, the same way our biochemistry mnemonics page does!
Oxidoreductases (EC 1)
Oxidoreductases catalyze oxidation-reduction (redox) reactions, moving electrons from one molecule to another. Most work with an electron carrier like NAD+, NADP+ or FAD. A name with dehydrogenase, reductase or oxidase gives one away!
Keep the electron flow straight with OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
In the citric acid cycle, isocitrate dehydrogenase oxidizes isocitrate to α-ketoglutarate, releasing CO2, and reduces NAD+ to NADH. The cycle has four of these redox steps, and glycolysis has one, run by glyceraldehyde-3-phosphate dehydrogenase.
Transferases (EC 2)
Transferases move a functional group from one molecule to another. Kinases are transferases that move a phosphate group from ATP, like hexokinase adding a phosphate to glucose.
Aminotransferases move amino groups, and peptidyl transferase links amino acids together in the ribosome. Once you know the group that moves, you’ve got the class!
Hydrolases (EC 3)
Hydrolases break a bond by adding water across it. Phosphatases, peptidases, lipases and nucleases are all hydrolases, and many are named for their substrate plus -ase, like ATPase, GTPase and peptidase.
From an organic chemistry view, hydrolyzing a peptide bond is a nucleophilic acyl substitution. Water’s the nucleophile, and it attacks the carbonyl carbon.
Lyases (EC 4)
Lyases break a bond without water and without oxidation. The reaction often leaves a new double bond or ring! Aldolase in glycolysis and the decarboxylases are lyases.
Run in reverse, a lyase adds a group across a double bond and joins two molecules into one. Lyases doing that job are often called synthases.
Isomerases (EC 5)
Isomerases rearrange the atoms within one molecule, so the product is an isomer of the reactant with the same molecular formula.
Triose phosphate isomerase and phosphoglucomutase are two examples, and names ending in isomerase, mutase or epimerase point to this class. They’re one of the easier ones to spot!
Ligases (EC 6)
Ligases join two molecules into one, powered by ATP or another nucleoside triphosphate. DNA ligase seals the gaps between Okazaki fragments on the lagging strand. Aminoacyl-tRNA synthetases are ligases too, and any enzyme named a synthetase belongs here!
Translocases (EC 7)
Translocases move ions or molecules across a membrane. The Na+/K+ ATPase pumps three Na+ out of the cell and two K+ in for each ATP, and ATP synthase makes ATP as protons flow back across the inner mitochondrial membrane.
Both were filed as hydrolases (ATPases) before 2018, so older practice material may still label them that way. Don’t let that throw you!
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How to Tell Similar Enzyme Classes Apart
A few classes get mixed up because their reactions look alike, and that’s normal! Ask yourself two questions. Is a bond being broken or made? Is water or ATP part of the reaction?
Synthase vs synthetase
A synthetase is always a ligase, since it spends ATP or another nucleoside triphosphate to join molecules. A synthase makes something without that energy coupling, and it can sit in another class.
Many synthases are lyases running in reverse, and ATP synthase is a translocase. Tricky, right?
Kinase vs phosphatase
A kinase (a transferase) moves a phosphate group from ATP onto a molecule. A phosphatase (a hydrolase) uses water to take a phosphate group off. They undo each other’s work, which is how cells switch many proteins on and off!
Name clues for each class
Cofactors and Coenzymes
Many enzymes need a small nonprotein helper in the active site before they can work. Without it, the enzyme is an inactive apoenzyme. With the helper bound, it’s an active holoenzyme, ready to go!
Cofactors are inorganic, usually metal ions from the minerals in your diet. Kinases need Mg2+ bound to ATP, and carbonic anhydrase needs Zn2+. They’re small, and they matter a lot!
Coenzymes are small organic molecules, and most are made from vitamins. Many act as carriers, picking up electrons or a chemical group in one reaction and handing it off in another. They’re reusable!
A helper that stays tightly bound, often covalently, is called a prosthetic group. The FAD attached to succinate dehydrogenase is one!
Notice how many coenzymes carry electrons? That’s why they pair with oxidoreductases, and our biological oxidation-reduction notes follow those electrons further.
Enzyme Classification Practice Questions
Give each one a try before you open the answer. You’ve got this!
Question 1. During DNA replication, the lagging strand is made in short Okazaki fragments, which leaves a gap between each fragment and the next. What type of enzyme joins the fragments into one continuous strand?
- Oxidoreductase
- Isomerase
- Hydrolase
- Ligase
Show the answer
D, ligase. DNA ligase forms a phosphodiester bond between two DNA fragments, joining two molecules into one bigger one. It’s powered by ATP (or NAD+ in bacteria), which fits the ligase pattern. Nice work if you got it!
Question 2. Which of the following is false?
- When an isomerase catalyzes a reaction, the product has one more carbon atom than the reactant.
- An enzyme with dehydrogenase in its name usually catalyzes the transfer of electrons between molecules.
- Hexokinase is an example of a transferase, and GTPase is an example of a hydrolase.
- The induced fit model of enzyme-substrate binding is better supported by research.
Show the answer
A. An isomerase converts one isomer into another. Isomers share a molecular formula and differ only in how their atoms are arranged, so the product has the same number and types of atoms as the reactant.
A product with an extra carbon can’t come from an isomerase.
B, C and D are all true. A dehydrogenase is an oxidoreductase. Hexokinase transfers a phosphate group from ATP to glucose, and GTPase breaks down GTP by hydrolysis. Induced fit accounts for the conformational changes during binding. Great job if you spotted A!
Question 3. Under the current EC system, which class does the Na+/K+ ATPase belong to?
- Hydrolase
- Translocase
- Transferase
- Lyase
Show the answer
B, translocase. The pump uses the energy from ATP to move three Na+ out of the cell and two K+ in.
Translocases move ions or molecules across membranes, and they’ve been their own class since 2018. Before that, the pump was filed as a hydrolase. You’ve got it!
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Key Terms for Enzyme Models and Classification
Enzyme Classification FAQs
What are the 7 types of enzymes and their functions?
Oxidoreductases move electrons (redox), transferases move functional groups, hydrolases break bonds with water, lyases break bonds without water, isomerases rearrange a molecule, ligases join two molecules using ATP, and translocases move ions or molecules across membranes. That’s the whole list!
What are the 6 classes of enzymes?
The six classic classes are oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases, remembered as LI’L HOT. Older textbooks stop at six, and since 2018 translocases have been the seventh class.
What’s the difference between the lock and key and induced fit models?
In the lock and key model, the active site already matches the substrate and nothing changes shape. In the induced fit model, binding changes the enzyme’s shape until it fits. Induced fit is the model research supports!
What are some common enzymes and their functions?
Amylase and lactase break down starch and lactose (hydrolases). Hexokinase adds a phosphate to glucose and DNA polymerase builds DNA (transferases). DNA ligase joins DNA fragments, and ATP synthase makes ATP (a translocase).
What’s the difference between a cofactor and a coenzyme?
A cofactor is inorganic, usually a metal ion like Mg2+ or Zn2+. A coenzyme is a small organic molecule, usually made from a vitamin, like NAD+, FAD or coenzyme A. Inorganic vs organic is the whole difference!
Is a synthase the same as a synthetase?
No. A synthetase is a ligase and uses ATP or another nucleoside triphosphate. A synthase makes a product without that energy coupling, and it can belong to another class, like the lyases or translocases. It’s an easy mix-up!
Final Thoughts on Enzyme Models and Classification
Two models and seven classes cover this whole topic, and you’ve just learned them all! Use induced fit for binding, read the reaction to name the class, and check whether water, ATP or a membrane is involved when two classes look alike.
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More Enzyme Study Notes
- Enzyme Structure & Function on the MCAT
- Enzyme Kinetics on the MCAT
- All Enzymes study notes
- The Enzyme Kinetics & Inhibitors cheat sheet, free with our MCAT Cheat Sheets
VII. Supplemental Resources
Want to go deeper on this topic? Here's where to read, watch and practice it next.
Read
- Kaplan MCAT Biochemistry Review 2026-2027, §2.1 and §2.2
Watch
- “Six types of enzymes” (Khan Academy)
- “Induced fit model of enzyme catalysis” (Khan Academy)
Practice passages
- “Lactate dehydrogenase and exhausted athletes” (question 1 of 5)
- “Biofilms physical properties in dental plaques” (question 3 of 6)
Official AAMC practice that tests this topic
- Bio/Biochem Section Bank, Passage 1, Q1
- Chem/Phys Section Bank, Q28
- Chem/Phys Section Bank, Q47
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